This is a phase I study for safety. It is a double-blind, randomized, placebo-controlled study comparing changes in GFR and urinary prostaglandins after administration of SC-58635 or naproxen to patients with stable CRF. Either SC-58635 200 mg. BID, naproxen 500 mg. BID or placebo is given for six consecutive days followed by a single day 7:00 am dose. Safety is assessed by physical exams, clinical labs, weight, blood pressure and pulse, and adverse signs and symptoms.